Gith would have been great! I suspect that it was too big of change - they would feel obligated to make a DLC. Because one guesses that Gith's imprisonment was part of the deal with Tiamat. And then the adventurers would need to take on the dragon queen . . .
Yeah this would be good too. And conveniently, Tiamat is I think the only God in 5E with a pre-existing statblock to set game design expectations.
